Re: [ATM] silvering vs mercury: Another Question: Silver removal

[Mark Holm <mdholm@telerama.com>]

Pete wrote:
> Hi all,
> Silver won't stick well to glass without an adhesive bonding layer.
> So, with chemical silvered mirrors, get some good sticky tape (clear
> packaging tape works well) and apply it to the mirror in small sections
> at a time and the silver will be gone very quickly.
> Then use acetone to remove any sticky glue residue from the tape.
> Finally just take a cotton ball with some polishing compound and water
> and lightly rub all over the surface to remove the last traces of silver.
> Hope this is helpful,
> Peter Nance, Jr.
> lenses@adelphia.net

I think the first sentence is incorrect.  Silver will stick to glass
with no adhesive bonding layer if the glass is sufficiently clean and is
kept wet with distilled water between the cleaning and silvering
operations.  I have never heard of anybody using an adhesive in the
usual sense to stick silver to glass.

As far as getting the silver off, I can't work up much enthusiasm for
using polishing compound.  You are certain to do some damage to the nice
smooth surface you worked so hard to get on the glass.  Silver is very
easy to dissolve off with either weak or strong nitric acid.  As for the
idea of using circuit board etchant, ferric chloride will convert the
silver to silver chloride.  Whether the silver choride will wash easily
from the glass, I simply don't know.  Silver chloride is not very water
soluble, but it may not adhere well to the glass, so it may rinse off
despite not being soluble.  Silver nitrate is quite soluble (as nitrates
often are), so the nitric acid treatment converts silver metal to a
solution quite reliably.  The nitric won't need to be very concentrated
for the job of dissolving silver.  Very strong (fully concentrated)
nitric acid also is a strong oxidizing agent for organic materials.  It
will burn off traces of oils, pitch residue, etc. that even acetone
won't fully remove.  I say, give the mirror a warm soapy water wash to
get the worst of any crud off.  Rinse it well several times, then blot
it dry and give it a couple of acetone washes to get rid of most of the
organic residues.  Then pour on a little concentrated nitric and swab
that around gently with a wad of cotton wool.  Then rinse several times
with distilled water (don't pour water into acid.  drain off the bulk of
the acid first), taking care not to touch the front surface with
anything.  This should result in a surface that is quite clean, except
for an adsorbed water layer, that you will have anyhow, and is a normal
part of the glass to silver interface.  Keep the glass under distilled
water until you pour on the silvering solution.  You don't want it to
dry out.  Dispose of the nitric soaked cotton wool in a large container
of water to dilute the nitric acid in it.  You don't want to leave it
undiluted, because it can catch fire, as well as being hazardous from
the nitric acid.

The only hang up in this procedure is working with concentrated nitric
acid.  It is nasty stuff, not to be mucked about with carelessly.  OTOH,
chemists who know what they are doing and have the appropriate safety
gear, wouldn't hesitate to do it.  They play with a lot worse stuff than
nitric acid on a pretty regular basis.

Appropriate safety gear for concentrated nitric acid:
	Long cuffed rubber gloves.
	Safety goggles & a full face shield.
	Rubber apron.

	The full face shield plus safety goggles double protection for your
eyes is partly in compensation for the fact that, in a home workshop,
you are unlikely to have an eyewash fountain or safety shower.  You
really, really don't want concentrated nitric acid in your eyes, and if
by accident it should get there, you really, really want to have a very
good eyewash fountain immediately available.  The other good safety rule
is to have a responsible person standing by who knows the correct drill
for the hazards involved.  The correct drill for nitric acid exposure is
flooding the skin or eye with lots of fresh water as quickly as possible
and lasting for at least 15 minutes, followed by a rapid trip to the
emergency room.  A person with a strong irritant in an eye will not be
able to open their eyelids voluntarily.  You have to pull them open by
hand to get the water in.

There may well be other methods for getting the glass clean enough that
don't involve concentrated nitric acid.  Of course, if they involve
chromic acid or nitric-sulfuric mixture, then they are even worse than
nitric from a safety point of view.  Even chemists think twice before
mixing up nitric-sulfuric.  (Appropriate thoughts: Do I really need to
do this?  Do I really know how to do it correctly?  What the hell am I
going to do with the stuff afterward?)

Mark Holm
mdholm@telerama.com

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